1. From standing, I grip his opposite lapel with my right hand like I'm setting up a collar choke. My left hand holds his sleeve at right elbow.

2. My right foot steps on his right hip and stretches him back to bend him over as...
3. I sit down, spinning my head counter-clockwise (so my hips spin to the left) and throw my left leg over his face. Make sure you pull his elbow in deep and lift your hips high.

4. I finish the armbar by squeezing my knees, pulling his head down with my left leg (like I want my foot to touch my butt) and lifting my hips. My hand comes off his collar and holds his wrist to make sure his arm stays extended and his thumb is pointing off my chest.

You're going for an omoplata on their right arm but they posture up and turn into you to escape.

You switch your legs and catch their head in a triangle. The only problem is that his arm is still in the omoplata position and you can't get a good angle for the choke. He may even grip his arms together behind your legs to stop you from moving your hips.

Slide your right hand against the inside of your right knee (next to his head) and switch your legs, bringing your left foot to your right hand and grabbing your ankle to keep his posture broken.

Reclose your triangle on this side. You've now got the triangle locked the "wrong" way, thus the "backwards" in the name.

But due to the position, you can use his right shoulder and your right thigh against his neck to submit him. Squeeze your knees together and hug around your knees to affect the choke. The finishing pressure isn't really how hard you lift your hips or pull on his head but how tightly you bring your knees/thighs together. This is true of normal triangles too, but even more so here.